1. Field of the Invention
The present invention relates to a scanning electron microscope and, more particularly, to a scanning electron microscope which can produce a high-resolution image even if the specimen is tilted when a retarding method in which a negative voltage is applied to the specimen is utilized.
2. Description of Related Art
In low-voltage imaging scanning electron microscopy, an electron beam is accelerated at a low accelerating voltage to avoid specimen charging and damage. However, as the energy of the electron beam passing through the objective lens decreases, the aberration tends to deteriorate. To circumvent this situation, one method has been put into practical use. In particular, the energy of the electron beam is increased and the beam is passed through the objective lens area. A negative potential is applied to the specimen to decelerate the beam immediately ahead of the specimen before the beam enters the specimen. This method is known as the retarding method and permits high-resolution imaging even at low accelerating voltages. In this case, secondary electrons emitted from the specimen are captured by the magnetic field produced by the objective lens and pass through the inner polepiece and then go to the top of the objective lens. Therefore, the secondary electrons are detected either inside or above the objective lens. In this retarding method, an electric field for decelerating the primary electron beam is produced between the specimen and objective lens. Where the specimen is not tilted, the field between the specimen and objective lens maintains an axis of symmetry with respect to the optical axis of the electron beam. Therefore, the effect (astigmatism) on the primary electron beam is small. Where the specimen is tilted, however, the primary electron beam is affected more. Scanning electron microscopy permitting high-resolution imaging even in this case has been discussed (see, for example, Japanese Patent Laid-Open No. 255588/1996).
In the above-described retarding method, when the specimen is tilted, the electric field between the specimen and objective lens is no longer axisymmetrical with respect to the optical axis of the beam. An electric field component that is lateral with respect to the optical axis is produced. Astigmatism in the primary electron beam increases. As a result, it is not possible to obtain high-resolution.